Recent years have seen the evolution of ideas for using the telephone network, as it is increasingly digitalised, as the basis for a worldwide Integrated Services Digital Network (ISDN). This has taken place pragmatically, as many different interests have been brought to bear on the problems. The result is a set of emerging recommendations which do not necessarily hang together rationally. These recommendations and their implementation in practical networks, will undergo further evolution before the ISDN concept achieves stability.
Where a telecommunication system has to provide more than one type of service, it is known to provide two or more different switching networks. Thus in British Patent Specification No. 1122924 (International Standard Electic Corporation) a system is described which handles both telephone and telegraph traffic. A first switching network with its own control means, and a second switching network also with its own control means, are provided. There is a further control means associated with both networks and serving the subscriber lines. The further control means co-operates with both the other control means as and when needed.
Another known arrangement is in British Patent Specification No. 1363329 (International Standard Electric Corporation). There an exchange serves subscribers some with video-phone equipment. A first switching network handles non-video calls and a second switching network handles video connections, the same control equipment being used for both networks. When a wanted number is received, the control equipment checks to see if both subscribers have video equipment, and if so sets up the call via the second network. If one or both subscribers does not have video equipment the call is set up via the first network.
Also of interest are British Patent Specifications No. 1363433 (Standard Telephones and Cables Ltd.), No. 1351497 (Licentia), No. 1344001 (Western Electric), No. 1297565 (Western Electric, No. 1108003 (Western Electric) and British Published Application No. 2132446A (Standard Telephones and Cables Ltd.)